There is a need for electrically passive fiber optic position sensors in aircraft systems due to the inherent advantages of fiber optics in view of the drawbacks of other kinds of sensors, particularly the active electrical. Passive sensors are compatible with fiber optic data links which may be a part of fly-by-light type aircraft. Linear variable differential transformers (LVDTs), the most commonly used electrical position sensors, have been proven to be reliable and are readily available with measurable position scales ranging from 0.01 to 10 inches. However, since they are electrically operated, LVDTs can easily malfunction when there is electromagnetic interference from such things as lightning, radio transmissions or microwaves. Another approach in the related art is the optically powering of LVDTs. The electrical power needed for the LVDTs is obtained by converting the optical power that has been transmitted through the fibers. The needed optical-to-electrical and back-to-optical conversions make such a system very inefficient.
Most fiber optic position sensors operate on the intensity modulation principle. The position of a (e.g., twelve bit) gray-coded encoder bar is read with read heads which provide analog or digital light pulses that are multiplexed either by time division or by wavelength division. A common problem in intensity-modulated fiber optic position sensors is contamination of optics which degrades performance and results in error in the decision-making electronics. An avoidance of the problem was attempted by utilizing an optically powered electrical position sensor such as an LVDT. Another problem is the potentially troublesome complex time division or wavelength division multiplexing of the intensity modulated sensor.